One common way of measuring the impedance of a circuit device is by injecting a known current into the device and measuring the voltage across the device. For DC measurements, a constant current source is relatively easy to construct for making DC impedance measurements. However, a variable-frequency constant current source is difficult and expensive to construct.
A current source can be constructed from a voltage source and a series impedance, where the value of the series impedance is much greater than the impedance of a device to be measured. However, this approach attenuates the measurement signal so much that it is difficult to measure low impedance devices of less than 10 ohms. This approach places an unbalanced load on the end of an output signal line, such as, for example, a 50 ohm coaxial line, from the variable-frequency voltage source. The ground and shield currents which are generated by this unbalanced arrangement are often orders of magnitude greater than the signals measured across the impedance under test.
Expensive commercial test equipment is available for measuring impedances at frequencies above 100 kHz. One such test equipment arrangement provided by the Hewlett Packard Company employs an HP4195A network analyzer which is connected to an HP 41951A impedance test set. An impedance device under test is mounted to the relatively expensive test set. The impedance test set uses a current sampling inductive bridge transformer arrangement by which input current is sampled in a primary winding and a secondary winding provides an output voltage which is proportional to the input current. The network analyzer provides test input signals to the impedance test set. Connections are made from the impedance test set to the network analyzer to provide measurement signals for the network analyzer to calculate the value of an impedance device under test. The impedance test set is limited to operation above 100 kHz, so that low frequency or DC impedance measurements cannot be made with this arrangement. Besides the limitations in lower frequency measurements, the impedance test set is expensive. The network analyzer is very expensive.
Consequently, a need exists for a relatively low cost impedance measurement system which is capable of measuring very low impedance values from DC to RF frequencies.